The invention relates to a method of encapsulating solder metal powders, especially the finest solder metal powders, in which the powder is provided with a thin polymer protective layer by a polymerization reaction carried out on the surface of the solder powder.
The invention relates further to a solder metal powder produced by this method and having a core of solder metal with a diameter of 1 to 100 xcexcm and a protective layer surrounding this core of polymer.
A method of this type for producing microparticles of a reflow soldering agent is known from DE 44 02 042 A1, in which the compact solder metal is melted in a high temperature organic liquid and is dispersed as spherically symmetrical particles in a particle size range of preferably 3 to 10 xcexcm in a diameter by a flow dispersing process and the organic liquid is removed to the extent that the metal particulate remains coated, so that it can be transferred into an emulsion and the individual particles of the suspension and emulsion coated with a melamine polymer to a layer thickness in the range of 50 to 250 nm by the complex coacervation method. The microparticulate organic phase is then quantitatively separated from the microparticulate metal phase.
This microparticulized metal powder is indeed protected by a thermosetting plastic polymer system but can be liberated only by the use of a very strongly activated flux. This flux can give rise to damage to microelectronic circuits and is thus unsuitable.
A solder powder is, in addition, known from EP O 556 864 A1 which is provided with a protective layer of parylenes. The parylene layer is applied by means of a vapor deposition whereby dimers are decomposed by pyrolysis to monomers, the monomers are condensed from the gas phase onto the surface of the powder particles and the monomers there undergo the polymerization reaction and film forming.
This known process has the disadvantage that for use of the particles in solder pastes, layers thicknesses of less than 100 nm is required and under these conditions there is no assurance that there will be complete encapsulation of all particles and the prior problem remains of a measurable permeability to small molecules which are detrimental to the solder powder like water, and gasses like oxygen and sulphur dioxide.
This known encapsulating material also has the drawback that it can be dissolved only with highly active fluxes. A use in soldering processes has thus not become known.
Faced with this state of the art, the invention has as its object the provision of a method and a microencapsulated solder which are improved in that the metal powder is reliably protected from external oxidative processes and simultaneously the encapsulation can liberate the metal powder without requiring fluxes and only by the effect of temperature.
This object is achieved by the following method steps.
a) producing a suspension of a powder and a hydrophobic liquid,
b) generating a hydrophobic surface layer on each metal particle by the addition of a cationic tenside [surface active agent] with a chain length of C1 to C20 with continuous stirring to form a brush structure on the hydrophobic layer of step (a),
c) stirring the mixture from steps (a) and (b) until a viscous homogeneous mass is formed,
d) admixing a radically polymerizable monomer to the mass in step (c) and which is capable of forming a thermoplastic polymer with a glass temperature Tg of at least 60xc2x0 C. below the solid temperature of the solder powder,
e) adding an organic initiator to start an interfacial polymerization reaction incorporating the hydrophobic layer of step (a) and forming a protective layer of thermoplastic polymer that has flux characteristics,
f) introducing the mass from step (e) into an aqueous system with continuous stirring, the system containing an emulsifier for suspension stabilization, and controlling the polymerization reaction by tempering [controlled temperature heating] to 50xc2x0 to 90xc2x0 C. and maintaining the mass at this temperature for at least 120 min, and
g) cooling washing and recovering the solder power encapsulated in steps e) and f).
According to a further preferred feature of the method of the invention, as the hydrophobic liquid, nondrying vegetable and/or animal oils and/or aromatic solvents are used. Preferably vegetable oils, ricinus oil, olive oil or peanut oil is used.
In a further feature of the method of the invention, quaternary ammonium salts, preferably dodecyltrimethylammoniumchloride or cetyltrimethylammoniumchloride metal chloride is used as the tenside.
A preferred feature of the method of the invention utilizes as the monomer methacrylic acid-2-hydroxyethyl ester or methylmethacrylate.
To initiate the polymerization reaction, organic peroxides or azo compounds are used as initiators.
Furthermore, the object is achieved with a microencapsulated solder powder in which the metallic core is formed with a brush like structure with a surface active hydrophobic liquid and a tenside and in which a thermoplastic polymer encapsulating wall with fluxing agent characteristics is anchored, whereby the microencapsulating wall has a glass temperature Tg of at least 60xc2x0 C. below the solidus temperature of the core.
The hydrophobic layer on the metal core is composed, according to further feature of a nondrying vegetable and/or animal oil and/or aromatic solvent, preferably castor oil, olive oil or peanut oil and a cationic tenside with a chain link of C1 to C20 whose length determines the layer thickness of the encapsulating wall.
Suitable monomers for forming the encapsulating wall are radically polymerizable monomers, preferably methacrylic-2-hydroxyethylester or methylmethacrylate.
The encapsulation according to the invention protects the powder properties. Simultaneously the sheath ensures protection against the influences of atmospheric effects like oxidation, moisture and cover layer formation. The oxidative attack on the metal particles within the paste by activators is also limited. As a result there is an enhanced stability against undesired chemical reactions in solder pastes made therefrom. The use characteristics in terms of processability and storage stability of the pastes are clearly improved.
The encapsulation walls on the inorganic particles gives rise to a substantially improved coupling of these particles to the polymer matrix in applications such as paste or dyestuffs which leads to a reduced sedimentation tendency and improved rheological characteristics.
The polymer used has a glass temperature, i.e. the temperature at which the polymer makes a transition into the plastic state, of 60xc2x0 to 70xc2x0 C. This ensures that the polymer encapsulating sheath will free the metal powder before it melts and will not hinder the soldering process. By the choice of monomers according to the invention the encapsulating material has fluxing agent properties.